As far as headspace goes in the carboy, I also ordered some 3/4" Food Grade Delrin (Acetal) Balls https://www.usplastic.com/catalog/item.aspx?itemid=126144 as an alternative to marbles, not always knowing the source of marbles and if they are lead-free, to lessen the headspace in the secondary. The Acetal balls are heavier than water and would sink in the carboy.
I had no idea Delrin was that dense! IMHO, 1.4 g/cc is pretty surprising for a simple plastic.